1. Field of the Invention
This invention relates generally to electrochemical cells and more particularly to a new and improved separator structure useful in an electrochemical cell having an anode, a depolarizer/liquid catholyte or a solid cathode/electrolyte system. Still more particularly, the present invention relates to the use of a separator structure in addition to the traditional or main separator in a spirally wound cell configuration. The separator structure of the present invention is provided to augment the primary or main separator disposed between the anode and the cathode electrodes to further help prevent internal electrical short circuit conditions from occurring.
2. Prior Art
In addition to providing a physical barrier between the anode and the cathode, the separator is usually porous in nature, provides high ionic conductivity and exhibits extremely high electrical resistivity. Presently, various types of woven or nonwoven paper, fiberglass, microporous materials and the like meet these criteria and are used as separator materials by those skilled in the battery art. However, many of the separator materials that are used in alkali metal electrochemical cells have very low tensile strength, especially when wetted with the electrolyte or catholyte material. Furthermore, when wetted, many of the separator materials become increasingly susceptible to tearing and puncture.
These latter characteristics can lead to internal short circuit conditions, especially in cells containing a spirally wound cell structure having the anode and cathode assemblies tightly wound at the center of the electrode stack. Because of the tight curvature of the electrodes, this area contains a relatively large degree of stress in comparison to the remaining portion of the wound electrode assembly. Generally, this centered area of high stress is limited to the first one inch length of the innermost electrode. Additionally, small areas of exposed electrode screen can sometimes be left uncovered by the electrochemically active anode or cathode materials. The exposed screen can have sharp edges or points often located at the leading edge of the electrode. The separator material can be weakened by the wetting electrolyte and is susceptible to tearing in this area where higher mechanical stresses occur. As such, the separator at the center region of the spirally wound electrode stack is susceptible to puncture by sharp strands of exposed electrode screen.
Besides maintaining an acceptable tensile strength even when wetted with an electrolyte, there are several other requirements which a suitable separator material must exhibit to be used in an electrochemical cell or battery. It must be chemically inert, resist corrosion and be able to withstand high temperatures for the intended application of the cell or battery. While woven and nonwoven fabrics, fiberglass, microporous materials and the like exhibit many of these features, many polymeric materials have also been demonstrated to be candidate separator materials. Particularly, tetrafluoroethylene-ethylene copolymer (PETFE) has been described in U.S. Pat. No. 5,415,959 to Pyszczek et al., which is assigned to the assignee of the present invention and incorporated herein by reference, as a material which is chemically inert to the battery materials used in alkali metal cells, is corrosion resistant and does not decompose at any temperature which the alkali metal cell or battery could be expected to remain operational. Additionally, PETFE is extremely resistant to tearing and puncture.
Although PETFE can be used as the sole separator in electrochemical cells or batteries and particularly alkali metal cells or batteries, the application of this material as a separator has drawbacks when applied in practice. The material is not as ionically conductive as the woven and nonwoven fabrics and fiberglass materials, and the relatively high cost associated with PETFE can substantially increase the cost of an electrochemical cell or battery.